The present invention relates to an inspection coaxial probe which establishes reliable connection between a device to be inspected and inspection equipment for inspecting an electrical characteristic of a module of high-frequency/high-speed circuitry (an analog high frequency is called a high frequency; a digital frequency dealing with a very short pulse width and a very short pulse interval is called high speed; and both frequencies are hereinafter collectively referred to as a radio frequency “RF”). Here, an RF encompasses an analog high frequency and a high-speed digital short pulse having a short pulse interval, wherein an iteration rate of a sinusoidal wave or pulses is 1 G H z or more.
For instance, the RF circuitry includes an amplification circuit or a mixer circuit, which are to be incorporated into, e.g., a portable cellular phone, or a filter circuit, or an IC. Such inspection is performed before the module or IC is incorporated into a circuit board.
More specifically, the present invention relates to a coaxial probe for use in inspecting an RF device which eliminates a necessity for soldering a device to be inspected; which can effect full electrical contact even at an RF range; and which can also establish connection with the device to be inspected through use of a coaxial structure, which is free from the influence of noise, even when electrode terminals of the device are arranged at a very narrow pitch of about 0.4 mm.
The present invention is also related to an inspection unit incorporating such a coaxial probe.
When an electrical characteristic of an RF device, such as a semiconductor wafer, an IC, or a module, is inspected, impedance will vary if, particularly, a contact condition of a terminal section is incomplete, which causes variations in a measured value. For this reason, inspection is performed through use of a jig such as that shown in FIG. 8. Specifically, in order to prevent interference of external noise, an RF circuit, which is a device to be inspected, is assembled into a module 50 by incorporating an amplification circuit and a mixer circuit into a metal housing. RF signal input and output terminals 51, 54, a power supply terminal 52, and a ground (earth) terminal 53 are provided on the back face of the housing. The RF circuit is inspected by electrically connecting the RF circuit to respective terminals of a wiring board 66 on which wiring for inspection purpose is laid.
An example shown in FIG. 8 adopts the following configuration. Specifically, the configuration uses contact probes, wherein each contact probe is constituted by inserting a spring and one end of a plunger in a metal pipe such that the plunger is projected outwardly by the spring and such that the plunger is withdrawn when depressed. Electrode terminals are connected together by placing Rf signal contact probes 63, power supply contact probes 64, and earth contact probes 65 within a metal block 61 for preventing influence of external noise. In order to diminish an inductance component, the RF signal contact probe 63 is formed into a short probe having a length of, e.g., about 2 mm. Even in the case of such a short probe, difficulty is encountered in diminishing a reactance component of the probe to 1 nH or less at an RF range. For instance, the probe of 1 nH yields impedance of 63 Ω at 10 GHz.
Therefore, there is employed a coaxial structure in which a dielectric tube is interposed between the RF signal contact probe 63 and the metal block 61 and the contact probe 63 serves as a core conductor and the metal block 61 serves as an outer conductor, to thus prevent an increase in impedance or intrusion of noise (see, e.g., Japanese Patent Publication No. 2001-99889A). In FIG. 8, reference numeral 67 designates a coaxial cable; and 68 designates a support for retaining the metal pipe provided around an outer periphery of the contact probe.
Even when an IC is inspected lead terminals of an IC and a wiring board are connected together by way of an IC socket formed from a leaf spring. Alternatively, as shown in FIGS. 9A and 9B, an IC socket 70 is formed of a block 71 made of plastic or ceramic, and contact probes 72 are analogous to those mentioned previously and arranged in a matrix pattern in association with electrode terminals 73a of an IC 73. The IC 70 is connected to the wiring laid on a multilayer wiring board 75 by way of the IC socket 70.
As mentioned previously, probes to be connected with respective electrode terminals are covered with a metal block. Further, RF signal contact probes are constituted in the form of a coaxial line, thereby preventing occurrence of reflection or attenuation of an RF signal, which would otherwise be caused by a contact probe or intrusion of external noise or delivery of noise to other electrode terminals. Thus, a jig for inspecting an RF device is constituted. FIG. 8 shows two RF signal contact probes 63 (for input and output), one power supply contact probe, and one earth contact probe, however, all the probes are actually formed in a large number. Further, in association with a recent increase in the degree of integration of an IC, there is a case where electrode terminals having an area of about 600/cm2 are provided in a matrix pattern. An IC having electrode terminals arranged at a narrow pitch of about 0.4 mm has come into existence.
In order to achieve predetermined impedance by the coaxial structure such as that mentioned previously, an outer diameter “d” of the contact probe serving as a core conductor and an inner diameter “D” of a hole of the metal block serving as an outer conductor must satisfy the relationship expressed by Equation (1) provided below, while a relative dielectric constant of a dielectric substance interposed between the core conductor and the outer conductor serves as εr.
                              Z          ⁢                                          ⁢          o                =                              60                                          ɛ                r                                              ⁢                      log                                                                      ⁢              e                                ⁢                      D            d                                              (        1        )            
Therefore, even when an attempt is made to realize a coaxial structure having an impedance of 50 Ω through use of a tube formed from polytetrafluoroethylene, which is known as a dielectric having a small relative dielectric constant on the order of 2.1, the inner diameter D of the hole formed in the metal block must be about 3.3 times as large as the outer diameter “d” of the contact probe. Thus, the contact probes may fail to be compatible with a device to be inspected which has a narrow pitch of about 0.4 mm or less, unless the outer diameter “d” of the contact probe is reduced to 0.1 mm or less.
However, the contact probe has a structure in which a spring and a plunger are inserted into a metal pipe. If the outer diameter of the contact probe is reduced excessively, the contact probe becomes very costly. Further, there is an anxiety that durability and reliability of the contact probe are lowered.